Recently on the SETI email discussion list there has appeared some good dialog regarding possible
detection of Earth TV broadcasts at ET locations. [These discussions were stimulated by the question: could beings on worlds 40 light years distant really be viewing I Love Lucy? -- Ed.] This is not a novel idea.
An older book titled Communication With Extraterrestrial Intelligence
(editor Sagan, 1973, MIT Press, ISBN 0-262-69037-3) mentions the possibility
of eavesdropping by ETI in the chapter Techniques of Contact. I have seen
this idea mentioned many times in other writings.

Discussion on the list has been centered on detection of UHF television
broadcasts, which in the US encompasses the band 470-806 MHz (minus the
astronomy allocation at channel 37, 608-614 MHz). Why has interest been
focused on UHF television?

Some possible reasons:

In the US, all analog television channels are split across three bands; low
VHF (54-88 Mhz), high VHF (174-216 MHz) and UHF. Maximum permitted EIRP is
100 KW for low VHF, 316 KW for high VHF, and 5 MW for UHF. Horizontal
polarization is the rule, with circular polarization being allowed at the
broadcaster's discretion. It would thus be possible to radiate up to 10 MW
EIRP from a single facility. The varied limits across bands help compensate
for differences in terrestrial propagation, in theory placing the UHF
broadcaster at parity with the VHF broadcaster.

Because UHF TV stations radiate more power, and because the UHF band is
nearest to the so called "microwave window" of 1.2-50.0 GHz, it would seem
to some that these broadcast signals have the best chances of being detected
at astronomical distances. There are hundreds of UHF TV stations in the US
alone, each radiating day and night, year after year. Could these act as
unintentional beacons to an interested ETI?

This writer's opinion is that there are far better candidates for Earth's
"unintentional beacons". Here are my reasons:

One goal of a TV broadcaster is to deliver modulated RF as cheaply as
possible to as many receivers as possible. Five MW of power at UHF
frequencies is not easy to come by. The typical way of doing so is to
generate 60-240 KW of power at the transmitter, then excite an antenna
having 10-14 dB of gain. The 60-240 KW of power needed at the transmitter
output is obtained with much difficulty, usually by generating a modulated
carrier at low levels and passing that through class-A vacuum tube
(klystron, IOT or tetrode) amplifiers with attendant low efficiencies.

Electricity is not free. The engineer responsible for design and/or
operation of a transmission facility would not think of squandering RF by
intentionally directing it into the sky. The TV audience makes their homes
near ground level. Gain of a TV broadcast antenna is obtained by
concentrating the radiation into one direction. From a bird's eye view, the
signal radiates in all directions, equally well North, East, South and West.
From a side view, the main lobe is carefully engineered to equally
illuminate receiving antennas on the ground, both close in and near the
horizon. What is often done is to aim just below the horizon, and design in
some null-fill to accomodate the close in receivers. From a typical
transmitting antenna height of 1000 feet, the desired beam tilt will be
something on the order of 0.5 degrees negative (that is, below horizontal).
Any energy radiated above the horizon is wasted, as far as the broadcaster
is concerned.

In an imperfect world, some energy does radiate above the horizon, but this
side lobe is much reduced in amplitude. Likewise, some of the energy
striking the ground reflects back into space, but it will be scattered and
specular.

There are better "unintentional beacon" candidates for study and
speculation. Consider SPASUR (space surveillance radar) at 217 MHz, or
varied other defense radars operated by the military (see the Raytheon home
page for the names of some of these systems). Perhaps even HARRP
transmissions (2.8-10.0 MHz at 3 MW+) may be detected at great distances,
although the frequencies employed are far from ideal.

Some of you might be interested to know that the US has already orbited what
amounts to a sensitive spectrum analyzer which has observed the Earth.
Project Blackbeard was established to study TIPP (trans ionospheric pulse
pairs), events possibly associated with lightning storms. Frequencies
ranging from 25-100 MHz are observed. Spectrograms may be viewed on the
project's home page (http://sst.lanl.gov/nis-projects/blackbeard). They show
strong RF carriers of Earth TV and FM broadcasts. Keep in mind that these
carriers have been detected from a satellite a few hundred miles above the
Earth, not from interstellar distances.

I do feel strongly that reception and demodulation of Earth television and
radio by ETI is still highly possible, though not from interstellar
distances. How?

Many people study extinct cultures such as the ancient Egyptians, just to
choose one at random. We know of the Egyptians by their writings and
artifacts. What if they had television and we could send a probe back in
time to capture a few dozen terabytes of their transmissions? We would
obviously know more about them than we now do. Radio and TV broadcasts can
quickly deliver massive amounts of information about the society which
generates them.

Somewhere in the galaxy, there might exist a society that has lasted for
many thousands of years. This race may feel confident that intelligence will
someday arise on a small watery planet orbiting a yellow star in their sky.
This culture prides itself in the accumulation of wisdom, and projects that
might take a few thousand years to bear fruit are conducted with the same
vigor as short-term projects. They therefore send robot probes to promising
worlds. If such a probe were to detect the presence of non-natural RF
emanating from it's target world, it would perform a relay transmission back
to the home planet. If ETI is going to watch our TV broadcasts, this is how
it will happen, in my opinion.

I hope that this stimulates further discussion along these lines, and I
welcome differing points of view.